For LCD devices, a classification based on the operating mode of liquid crystal molecules includes modes such as PC (phase change), TN (twisted nematic), STN (super twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated bend), IPS (in-plane switching), VA (vertical alignment), FFS (fringe field switching) and FPA (field-induced photo-reactive alignment). A classification based on the driving mode of the device includes PM (passive matrix) and AM (active matrix) types. The PM types are classified into static type, multiplex type and so forth, and the AM types are classified into TFT (thin film transistor) type, MIM (metal-insulator-metal) type and so forth. The TFT type is further classified into amorphous silicon and polycrystal silicon types. The latter is classified into a high temperature type and a low temperature type depending on the production process. A classification based on the light source includes a reflection type utilizing natural light, a transmission type utilizing a backlight, and a semi-transmission type utilizing both natural light and a backlight.
The LCD device includes a liquid crystal composition having a nematic phase. The composition has suitable characteristics. An AM device having good characteristics can be achieved by improving the characteristics of the composition. Table 1 below summarizes the relationship between the two groups of characteristics. The characteristics of the composition will be further described on the basis of a commercially available AM device. The temperature range of nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of nematic phase is about 70° C. or higher and a desirable minimum temperature of nematic phase is about −10° C. or lower. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying moving images on the device. A response time that is one millisecond shorter than that of other devices is desirable. Thus, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1Characteristics of Composition and AM DevicesNo.Characteristics of CompositionCharacteristics of AM Device1Wide temperature range ofWide temperature range innematic phasewhich the device can be used2Small viscosity 1)Short response time3Suitable optical anisotropyLarge contrast ratio4Large positive or negativeLow threshold voltage anddielectric anisotropysmall electric power consump-tion, Large contrast ratio5Large specific resistanceLarge voltage holding ratio,and large contrast ratio6High stability to UV lightLong service lifeand heat1) A composition can be injected in an LCD device in a shorter period of time.
The optical anisotropy (Δn) of the composition relates to the contrast ratio of the device. A large optical anisotropy or a small optical anisotropy, namely a suitable optical anisotropy, is needed, depending on the mode of the device. The product (Δn×d) of the Δn of the composition and the cell gap (d) of the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on the type of the operating mode. This value is in a range of about 0.30 μm to about 0.40 μm for a device of a VA mode, and in a range of about 0.20 μm to about 0.30 μm for a device of an IPS mode or an FFS mode. In these cases, a composition having a large Δn is desirable for a device with a small cell gap. A large dielectric anisotropy (Δ∈) of the composition contributes to a low threshold voltage, low power consumption and a large contrast ratio of the device. A large Δ∈ is thus desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. It is thus desirable that a composition has a large specific resistance at a high temperature as well as at room temperature in an initial stage. It is desirable that a composition also has a large specific resistance at a high temperature as well as at room temperature, after it has been used for a long time. The stability of the composition to ultraviolet (UV) light and heat relates to the service life of the device. The device has a long service life when the stability is high. Such characteristics are desirable for an AM device used for a liquid crystal projector, a liquid crystal television and so forth.
A liquid crystal composition including a polymer is used for an LCD device of a polymer sustained alignment (PSA) type. First, a composition to which a small amount of a polymerizable compound has been added is poured into a device. Next, the composition is irradiated with UV light, while a voltage is applied between the substrates of the device, to polymerize the polymerizable compound and give a network structure of a polymer in the composition. In this composition, the polymer makes it possible to adjust the orientation of liquid crystal molecules, and thus the response time of the device is decreased and image burn-in is reduced. Such effects of the polymer can be expected for a device having a mode such as TN, ECB, OCB, IPS, VA, FFS or FPA.
A composition having a positive Δ∈ is used for an AM device of a TN mode. A composition having a negative Δ∈ is used for an AM device of a VA mode. A composition having a positive or negative Δ∈ is used for an AM device of an IPS mode or FFS mode. A composition having a positive or negative Δ∈ is used for an AM device of a PSA mode. Examples of the liquid crystal composition having a negative Δ∈ are disclosed in the following Patent document No. 1.
Patent document No. 1: JP H03-503651 A (1991).